The ability to detect and image structure that is below the surface of a sample is desirable in many applications. For many years, x-ray imaging was the most widely used method of imaging sub-surface structure; however, x-ray imaging has many drawbacks—particularly in medical applications—such as poor image resolution and increased risk of cancer from large-dose and/or cumulative exposure. These drawbacks have fueled development of alternative sub-surface imaging methods, such as optical coherence tomography (OCT).
OCT has rapidly become a critical diagnostic tool in areas such as biological, biomedical, medical screening, and vision-care. It utilizes low-coherence optical interferometry to enable non-invasive imaging of micron-scale microstructure inside biological tissue without subjecting the tissue to damaging ionizing radiation. In addition, its relatively low cost, high-resolution, and in-vivo capabilities make it an increasingly attractive imaging method. In the vision-care arena, for example, OCT is used to non-invasively image the human eye fundus, thereby facilitating diagnosis of retinal pathologies, such as macular degeneration, glaucoma, retinitous pigmentosa, and the like.
Early OCT systems were time-domain systems based on a relatively simple implementation of a free-space Michelson interferometer, in which an input light signal is split into a reference arm and a sample arm. In the reference arm, light is directed toward a movable reference mirror, which continuously reflects light back toward the detector as the mirror is moved along a scan length. The instantaneous length of the reference arm depends on the instantaneous position of this mirror. In the sample arm, light is directed into the sample at a first object point. Light is reflected back from the object point only by structural discontinuities, such as sub-surface structural features. The return signals from the reference and sample arms are combined at a beam combiner to form a signal that generates an interference pattern at a detector. Light that travels the same length in each of the reference arm and sample arm constructively recombines to form high-intensity signals that correspond to the depths of surface and sub-surface structural features at the first object point. This one-dimensional axial scan of the depth of an individual object point is typically referred to as an “A-scan.” By performing an A-scan at each of a plurality of object points, two- or three-dimensional images of the structure of the sample can be developed.
Unfortunately, while early time-domain OCT techniques were promising, their complexity and time-intensive nature served to limit their widespread adoption. As a result, alternative Fourier-domain OCT approaches were developed including Swept-source OCT (SS-OCT) and spectral-domain OCT (SD-OCT), which enable faster imaging with improved sensitivity.
In SS-OCT, the interferometric signal is sampled by a detector as a function of wavelength rather than mirror position. Typically, an object point in a sample is interrogated with a light source that sweeps through a range of optical frequencies (i.e., a swept source). As a result, the object point is illuminated with a beam of monochromatic light whose optical frequency is a function of time. This results in an interferometric signal of intensity versus wavenumber, k (k is proportional to the inverse of wavelength). A mathematical algorithm, referred to as a Fourier transform, is then used to convert the interferometric signal to a plot of intensity versus depth.
In SD-OCT, an object point is interrogated with broad-spectrum light. Light reflected from the object point is dispersed by wavelength along a row of detectors, which simultaneously provide a different output signal for each of a plurality of wavelength components. As a result, information is collected from many depths within the object point at the same time, and a Fourier-transform operation can be used to convert this information into a plot of intensity versus depth.
Historically, the interferometer portion of most OCT systems is based on bulk optics, fiber optics, or a combination of the two, making such OCT systems relatively large and expensive. Interferometers based on integrated optics offer a way to reduce the size and cost of OCT systems, however.
Integrated optics is a well-known technology wherein optical waveguides are formed on the surface of a substrate. These “surface waveguides” typically include a core or a first material that is surrounded by a second material having a refractive index that is lower than that of the first material. The change in refractive index at the interface between the materials enables reflection of light propagating through the core, thereby guiding the light along the length of the waveguide. Arrangements of surface waveguides, commonly referred to as planar lightwave circuits (PLCs), enable routing of optical signals within a small area, as well as complex optical functionality that can be difficult to achieve in free-space or fiber-optic-based systems.
Examples of OCT systems comprising PLC-based interferometers are described by Akca, et al., in “Toward Spectral-Domain Optical Coherence Tomography on a Chip,” IEEE J. of Sel. Topics in Quantum Elect., Vol. 18, pp. 1223-1233 (2012) and “Miniature spectrometer and beam splitter for an optical coherence tomography on a silicon chip,” Optics Express, Vol. 21, pp. 16648-16656 (2013), by Nguyen, et al., in “Integrated-optics-based swept-source optical coherence tomography,” Optics Letters, Vol. 36, pp. 1293-1295 (2011) and “Optical coherence tomography imaging with an integrated optics spectrometer,” Optics Letters, Vol. 37, pp. 4820-4822 (2012), and by Yurtsever, et al., in “Ultra-compact silicon photonic integrated interferometer for swept-source optical coherence tomography,” Optics Letters, Vol. 39, pp. 5228-5231 (2014).
While these PLC-based interferometers enable OCT systems that are significantly smaller than bulk-optic-based OCT systems, they require components (e.g., directional couplers, etc.) that have significant wavelength dependence. This can degrade system performance and reduce signal-to-noise ratio (SNR) and/or contrast-to-noise ratio (CNR). Further, the Michelson or Mach-Zehnder interferometer arrangements described typically exhibit large chromatic dispersion and polarization mismatches between their sample and reference arms. While these mismatches can be compensated in optical-fiber-based interferometer systems, compensating them in a PLC-based system can be quite challenging.
As a result, there remains a need for a PLC-based approach to sub-surface imaging that enables good system performance without significant wavelength dependency.